1. Field of the Invention
The invention relates generally to discharge lamps. The invention relates more specifically to novel starting aids for discharge lamps. The invention also relates to novel methods for making discharge lamps with novel starting aids.
2. Related Art
It is well known in the discharge lamp art that igniting a plasma discharge can be difficult. For most discharge lamps, the fields required to achieve ignition of the plasma are much higher than the fields required to bring the lamp up to full output and thereafter maintain a stable discharge.
Many patents describe different devices and methods for assisting the starting of discharge lamps. The prior art considered most relevant to the present invention includes U.S. Pat. No. RE32,626 and its related Japanese patent publication nos. 57-55057, 57-152663, 57-202644, and 58-5960. These publications disclose a relatively thick (e.g. 0.5 to 1 mm diameter) wire encapsulated in quartz and disposed inside an electrodeless lamp bulb for enhancing starting fields. However, numerous problems occur with the use of a thick wire inside a discharge lamp envelope. For example, it is difficult to protect the wire against the heat and reactivity of the plasma. A thick wire does not readily conform to the envelope wall, thus compounding the difficulty of protecting the wire from the plasma. Also, a thick wire blocks an appreciable portion of the light output and may even cast an undesirable shadow. All of the disclosed configurations are believed to suffer from significant coupling of energy to the starting wire which results in distortion of the plasma and eventual overheating of the wire.
One object of the invention is to provide field enhancement inside a discharge lamp envelope during starting to aid in the breakdown of an inert gas disposed as a fill material inside the envelope. An advantage of the invention is that for the same applied field such breakdown may be achieved at fill pressures which are higher than can be achieved without the present invention. A corresponding advantage is that a fill at a given pressure may be broken down at significantly lower power levels. While the inventors do not wish to be bound by theory of operation, it is believed that the present invention also provides advantages of increased lamp efficiency, reduced start and re-strike times, longer lamp life, and reduced stress on the RF source. Other potential advantages are believed to include bulb ignition without the need for external ignition devices, improved light output and/or spectrum using fills which would otherwise be difficult to ignite, reducing the envelope wall temperature by using low thermal conductivity gases (higher atomic number), and providing xe2x80x9cinstant onxe2x80x9d lighting by utilizing fill materials which are always in a gaseous state (e.g. SO2 gas). Another advantage is believed to include ignition of the inert gas without the use of radioactive starting aids (e.g. Kr85). Of course, discharge lamps utilizing principles of the invention will not necessarily provide all of the foregoing advantages, depending on the particular configuration and application.
One aspect of the present invention is achieved by a lamp bulb which includes a light transmissive envelope and at least one conductive or semi-conductive fiber disposed on the light transmissive envelope, where the at least one fiber is of a suitable material and is disposed in a suitable orientation to provide an enhanced starting field (e.g. a higher electrical field strength during starting). For example, the fiber may comprise a material or combination of materials selected from the group of carbon (e.g. graphite), silicon carbide (SiC), molybdenum, platinum (Pt), tantalum, and tungsten (W), and preferably has a thickness of 100 microns or less and may even be of sub-micron thickness. Aluminum may also be used, but is not preferred with quartz envelopes because aluminum reacts with SiO2 and causes devitrification. For example, the envelope encloses an inert gas and the fibers are effective to enhance a field applied to the gas to initiate a breakdown of the gas.
The light transmissive envelope may be made of any suitable material including, for example, quartz, polycrystalline alumina (PCA), and sapphire. Quartz is generally preferred for low cost applications.
The use of an extremely fine fiber as opposed to a relatively thick wire has many potential advantages, depending on the application. For example, the fiber is generally flexible and readily conforms to the bulb wall, thus keeping the fiber out of the steady state plasma discharge. Preferably, the fiber is coincident (i.e. in thermal contact) with the bulb wall along substantially its entire length (although a coating or adhesive may be between the fiber and the bulb). Without being limited to theory of operation, the fiber may be configured with relatively high resistance during steady state operation such that energy coupled to the fiber does not generate a significant amount of heat and any heat generated is readily dissipated because the fiber is heat sunk to the bulb wall. Without being limited to theory of operation, the fiber is believed to be relatively elastic as compared to a thick wire and therefore less susceptible to thermal stresses caused, for example, by different coefficients of thermal expansion. The fiber is practically invisible to the eye and thus does not block an appreciable amount of light output or cast a noticeable shadow.
Preferably, the fiber is disposed on an inside surface of the light transmissive envelope. The fiber may optionally be covered with a protective material to inhibit interaction between a lamp fill and the fiber. For example, the protective material may comprise a sol-gel deposited silica coating. For example, the protective material comprises a silicon dioxide coating less than 2 microns thick.
According to another aspect of the invention, a plurality of conductive or semi-conductive fibers are disposed on the lamp envelope.
According to another aspect of the invention, the fibers include silicon carbide whiskers.
According to another aspect of the invention, the fibers include platinum coated silicon carbide fibers.
According to another aspect of the invention, the fibers comprise a plurality of closely spaced parallel fibers. Alternatively, the fibers comprise a plurality of randomly distributed fibers. For example, each of the fibers is about 3 mm long or less.
According to another aspect of the invention, a discharge apparatus includes a light transmissive container having a light emitting fill disposed therein; a coupling structure adapted to couple energy to the fill in the container; a high frequency source connected to the coupling structure; and at least one fiber disposed on a wall of the container, wherein each of the fibers has a thickness of less than 100 microns, wherein the fibers are made from a conductive material, a semi-conductive material, or a combination of conductive and semi-conductive materials. The fibers are sufficiently flexible to readily conform to the wall of the container. For example, the fill includes an inert gas and the fibers are effective to enhance a field applied to the gas to initiate a breakdown of the gas. For example, the fill comprises a noble gas at a pressure greater than 300 Torr, the field applied to the bulb during starting is less than 4xc3x97105 V/m, and the applied field is effective to cause a breakdown of the noble gas.
In some examples, the high frequency source comprises a magnetron and the coupling structure comprises a waveguide connected to a microwave cavity. Preferably, at least one fiber is aligned with the electric field during starting. The apparatus may be a lamp and the container may comprise a sealed electrodeless lamp bulb. For example, the electrodeless lamp bulb comprises a linear bulb and the fibers comprise a plurality of fibers concentrated at respective ends of the linear bulb.
According to another aspect of the invention, a method of making a discharge lamp bulb includes providing a light transmissive envelope; and securing a fiber on a wall of the envelope. For example, securing the fiber comprises patterning the fiber on the wall with photolithography. Alternatively, securing the fiber comprises depositing the fiber inside the envelope and adhering the fiber to the wall of the envelope with a sol-gel solution. The method may further include covering the fiber with a protective material. For example, the protective material comprises silica and the covering comprises coating the fiber with a sol-gel solution.
The foregoing and other objects, aspects, advantages, and or features of the invention described herein are achieved individually and in combination. The invention should not be construed as requiring two or more of such features unless expressly recited in a particular claim.